Parallel transmission in MRI systems has become a topic of considerable interest during the last few years. The use of multiple individual radio frequency (RF) transmit coils is used to overcome B1 homogeneity limitations and to improve multi-dimensional RF pulses by shortening their duration. Of special concern in all these transmit applications is the specific absorption rate (SAR), which has to be kept below certain limits to avoid excessive patient heating. Different approaches have been discussed to reduce SAR in parallel transmission. The degree of freedom in RF pulse design allows selection of solutions with minimal SAR, e.g. via regularization techniques helping to enforce low SAR. Furthermore, the interplay between k-space trajectory and RF waveform can be used for SAR reduction (e.g., such as in a variable rate selective excitation, or VERSE, technique). The optimal RF pulse thus obtained is then used for the corresponding MR scan. In that respect, the parallel transmission RF pulse is optimized almost independently from the MR signal sampling process. The SAR limitation, particularly at high fields, is a serious problem in MR.
The present application provides new and improved systems and methods for reducing SAR in MR examinations, which overcome the above-referenced problems and others.